I noticed that if you upload a large pic (in data size, not pixel size) it takes a bit to load which I would expect.
Is there an optimum size / dimension for pics for the new forum software?
640x480 is normally the size to post in forums, some will post 800x600 or larger as you did. I posted a 4000x3000 picture as a test, it took 30-40 seconds to upload (with high speed cable access). It appears that the new forum automatically resizes photos to fit, so the only issues will be the upload and download speeds. I like the quality of the photos my camera takes at 12MP so I'll probably resize before I post, although I will also take some at 5 and 8 MP to check quality and upload speed.
Almost all digital images are accepted from a size perspective, but you're right, larger ones will take a bit longer to upload. As Ak1 mentioned, 640x480 is definitely a good size and you could even go a bit larger.