Best Image Size, Pixel Dimensions, and Format for Blog Uploads

I'm looking to upload images to my blogs. What is the "ideal" size/pixel/format should images be in?

That depends on the content of those images. For photographs or other continuous tone pictures, JPEG is best. For line art like schematics, PNG is good. GIF can also be good for that, especially if the diagram is black+white, and you tell the software to only use gray levels in the GIF.

As for pixel dimensions, I would make them as small as they can be to clearly show what you want to show. Larger pictures cause trouble or are inconvenient in some viewers. It's best if the picture doesn't need to be scrolled or the viewer resize it to fit into whatever window the user chose.

I would stay to 800 wide or less if at all possible. 512 or less is pretty much safe and you don't have to worry about it. If you really need more resolution (this is actually quite rare), then you can make the 512-wide picture clickable so that it brings up the full resolution version.

Thank you Olin. I just posted a blog and the two pictures I uploaded are larger than I'd like. I used 640/480 jpegs. I tried resizing with paint.net and the images did not shrink very nicely. I'll try the 512.

I don't know how smart paint is about resizing images. To do it properly you need to do a 2D convolution by a reasonable kernel of the nearby input pixels to a output pixel. Simply selecting the nearest pixel is going to create aliasing and look crappy. I use my own IMAGE_xxx utilities for that, including IMAGE_RESIZE. There must be lots of freely available stuff out there to do it properly.

If you're going to resize a image, it's always best to start with the original, not a previously resized image. So don't shrink your 640 image to 512. Go back to the original and shrink that to 512.