When I upload an image, it is available in the original size only. No alternate sizes are created/available within WordPress or in the file system. No error messages are given.
Troubleshooting Info: Working in an up-to-date plain vanilla install of WordPress 3.5.1, using the default theme with no plugins and media sizes are set at the defaults (150/150, 300/300, 1024/1024), on a server running Ubuntu 12.4.2 LTS, with the GD graphics library installed.
All other WordPress functions are working fine. Inserting images works fine, it just offers only the original size image, labeled "full size."
After initial testing, I then installed two image regeneration plugins: Regenerate Thumbnails, Simple Image Sizes. When I ran them without changing the default image size settings, they ran and announced completion/success but did not create any alternate image files. (Each image is still in its original uploaded size.)
Speculation: It is as if the alternate size information is not available to any image manipulation tool, either built-in or plugin.