8. Q: It uploads without any errors, but it does not generate thumbnails! The images show up full size on the page, or they are scaled in a really ugly manner.
A: In all cases I have seen this, the problem turned out to be with the image itself. WordPress uses the normal PHP functions for working with images, which are in the "GD" library. GD has problems with certain types of images. One particular image type that it cannot handle is grayscale images. If your image is black and white or grayscale, then make sure that it's a FULL COLOR image. That is, 24 or 32 bits per pixel. IrfanView is particularly good at converting images up from 256 color grayscale to 16 million color images. Just load the image up, select Image->Increase Color Depth, then resave it. Then upload the image.
Sadly this advice on the sticky has not helped much. Numerous times now I've tried to upload .JPG images and been greeted either by a confusing page full of "imagestring" or some such errors, or most recently, 'HTTP errors', whatever they may be.
I don't have these problems with the .PNG format, and am baffled as to the cause. The majority of my images are collected in Google Picasa, then used as-is or exported as .JPGs. I've tried using various degrees of compression, opening these images up in Photoshop to check the colour depth, and can see no obvious answers. Most misleading of all, this only affects certain images. If I take two straightforward scans, for example, one may upload without issue but the other will fail to generate any thumbnails.
Why is WordPress the only platform to have trouble with these images? I'm afraid it's a bit of a platform-killer for me; a shame, considering its themes and widgets are so useful.