Make your images 1024x512 pixels or smaller in that ratio i.e. 512x256, 256x128 etc. I save mine as .jpg
Upload them to a file hosting service or web space you have. If a file host, make sure they give instant access to the files and NOT one that makes you wait to download it.
In track mania place a sign block and choose the painter icon to choose what type of sign it is. Now, instead of choosing a sign
choose 'Select URL' and then type in the link to the sign from your web host.
The full link is needed ie 'http://www.webhost.com/signs/myimage.jpg' or whatever.
Validate the track and save it, and the sign will be downloaded for everyone ingame from then on.
For bigger images, just chop your picture up and use more signs
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