Also couldn't find anything regarding image limitations myself, so I don't think there are any real limitation.
Except that you have to be Jr. Member to post images if I'm correct.
Yes, you must be a Jr. Member first before posting an image as theymos says here
When the poster becomes a Jr member, the images that he tried to post previously will automatically be re-enabled
And with regards to the size of the image I tried to search it but there's no result and checking out the
Bounties (Altcoins) section where there are so many image on the threads. I guess there's no exact size limit on the image for each thread that you can do. You can check it and compare your image sizes.