What is the size limit of posted images?

The limit of allowable upload sizes depends on your Ranking here at RCT*Mart.

Your own limit should be indicated to the left of the "Browse" button where you'd attach uploads.

No I mean is there a limit to the dimensions of hotlinked images?

I don't know for sure the standards here, but most websites ask that you use a image no larger than 640 X 480, this allows those with smaller monitors to see the entire image without without having to scroll and helps make the forums neater in apperance too. The 640 X 480 is pretty standard.

^^^ What he said.

Most forums would prefer that you post/hotlink images that are no larger than 640 x 480 in size. 800 x 600 may also be acceptable. The smaller the image size without losing detail, the better.

Now, I'm not suggesting that you resize your images if they are larger, just try to not create the larger ones to begin with.