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I have these same issues:
The generated images are of the exact same size than the original image.
No errors are reported, the folders and the images are created without problem.
When I upload a new image, it saves a duplicate in the “thumb” folder, not resized and with identical file name (_thumb is not appended). “thumb” folder has 777 permissions. Resize Type is set to “constrain”.
Initial images uploaded are 108px wide & about 150px high. Thumb size is constrained to 54px wide, 200px high.
I can manually create a thumbnail by uploading the same file a 2nd time, renaming it by adding _thumb, then editing it to the thumbnail size.
I’m running on PHP 5.3.3 with GD enabled, Apache server.