This 'problem' arises
a lot. Images that work absolutely fine online suddenly do not work offline.
This is the result of one or both of these problems:
> The image not being cached by Graal.
> The image not being indexed(the biggest culprit).
Here I will guide you through steps you can take to fix these problems as to avoid having to ask a question that someone will have to answer for the hundredth time.
How to make sure Graal caches the image
This isn't exactly caching... but anyways... Graal has a file named FILENAMECACHE.txt in the Graal folder. What Graal does is it keeps track of all files handled by Graal(via downloading, saving or so on) and stores them in this file. This tells Graal exactly where to find the files to use them, so it doesn't have to sit and scan the whole folder(which gets HUGE over time) every time it starts up. However, if Graal(or one of its tools) did not have a direct hand in putting a file in its folder, Graal will never add it to FILENAMECACHE.txt, thus
it will never know it's even there. So you sit and make a tileset image and throw it in the Graal folder and are a little baffled why it doesn't work. You have two options to resolve this issue:
1) The easy way.

Whenever YOU add a file to the Graal folder, delete FILENAMECACHE.txt from the Graal folder(where Graal.exe is located), and start Graal. You will see a pop up notifying you that Graal is scanning its folder. After that's done it will have rescanned its folder fresh, thus will know your new file is there. Reopen whatever tool you were attempting to use and all should be fixed.
I suggest just doing this method!
1) The hard way.
This way is a little bit 'harder', but can be quite useful. It's not harder in the mindset that you will always be doing something complicated, but that it takes a bit more of a set-up. You have to associate your various Graal file types with their correct tool. For example, .nw with Graal Editor, and .gani with Graalshop. If you have not done so yet, the steps are quite easy. If you already have graal filetypes associated to their tools, you can skip past the next few steps.
Try to open the file. If it has no association, it will prompt you:

Tell it that you will select a program.
Chances are, Graal programs will not be there, so click browse(will look different between Windows versions):

Find your appropriate tool(Graalshop for this gani I am trying to open):

Tell Windows to always associate this program for this file.
Now whenever you open a file for Graal it will open directly in the right tool. This is essential, because whenever you do this Graal will do a temporary cache of the folder you loaded the file from. So if you're working on a gani, all you have to do is keep the images needed for the gani in the same folder as the gani. I do this often when I'm working on larger projects, as I keep the whole project organized in a folder on my desktop instead of clogging my Graal folder up with tons of folders.
Make sure the image is indexed
This is such a common occurence. The problem is, as Graal got updated, the tools fell out of date. As it is now, Graal itself supports a multitude of image types from bmp to jpeg, and various bit depths. However, the tools never got updated. So while images work fine online, you try to use them offline and the tools just don't support them. This is usually in relation to PNG's. PNG's support various bit depths, and Graal supports up to 32-bit(alpha blending and transparency). The Graal tools, however, support only 8-bit, or indexed. That means they can only support a PNG if it has been properly saved(the colors used stored in a 256 color palette).
MSPaint can not do this. MSPaint can indeed save PNG's, but it does not index them, nor give you the option to. Thus you'll need to use another program to do so. I only have Photoshop so that's all I can offer help with(it's all the same from various versions. I used to have Photoshop 6.0 and the method is exactly the same in Photoshop CS3).
Open the image in Photoshop. Navigate to
IMAGE > MODE > INDEXED COLOR

You'll get a window like this:

Fill it out pretty much like in the picture. Palette = exact. Forced = none(it won't force any colors into the palette). Transparency can be checked or not depending on if the image has transparency in it. I typically just leave it checked regardless, unless I'm running low on colors(like in a tileset. Remember, you only have 256 colors... more than that and you start losing colors. This is bad). Then just click OK and save your image. It should now work! I suggest keeping an unindexed copy of images that push the color limit(tilesets) in case you save an indexed copy and lose data. Because remember, you can still use the unindexed, or real version, online.
Hopefully this helps people having problems. If you still have problems after following these steps then feel free to make a post about it. Chances are those two steps above will fix any problems you have with your images/tilesets working offline.