what's the largest size image i can render without going over 512kb to post in Renderosity gallery?

You can render any size. I generally use 800x600. The trick is to use something like Photoshop, etc, to compress the .jpg image. The 800x600 at 72dpi/ppi works well at a compression of 10. I make my thumbs 180x whatever and compress to 5 in PS.

I have heard if you use Photoshop, to use the Image Ready part of Photoshop to do it. Straight Photoshop doesn't save .jpg's very well. BUT, this is just what I have heard. I use Photoshop, but use a completely different program to make .jpg's. .Jpg's can be tricky and there is no real answer to your question since the .jpg format will save ten different 800x600 pics all at ten different Kb's because each pic is different in what and how many colors it holds. Just experiment by saving, changing setting, saving, etc, etc. *A lot of programs will use a percentage to regulate jpg compression rate, I would keep it between 80%-95%. AgentSmith

It depends on the complexity of the image. Images that are easy to compress can be rendered at a higher res than noisy images with a lot of color-changes and sharp contrast and complex shapes. But most images should do fine for at least up to 1280x1024 pixels. PSP has a preview option that shows the original image and the compressed one next to each other with filesizes already calculated. UFX also has an option like that and Corel Photopaint has it too. I think most popular packages do that.

Yes PhotoPaint has that feature, you can also zoom and drag the preview window around to check on what might be areas of potential trouble with the selected compression. It also lets you know the resultant file size before you do it.