It's most likely because the image files are too big. So if you are on a dial-up, the connection times out.
What you can do is resize the images before you upload them. That way, the files will be smaller and the upload will go through faster. That would minimize the chances of getting a timeout.
For example, the original image of your screen that you posted above is 3648px × 2736px (1.6MB). On the board, it gets automatically resized down to to 600px × 450px (22.61 KB). So there is no need to upload such large file in the first place, since it's going to be resized anyway.
If you resize it before uploading, the actual file size for upload would be around 70 times smaller. That means the upload would complete 70 times faster, and there wouldn't be any problems with timeouts.