Can somebody point me to sites with not supported by Qt image format plugins (reading/writing). XCF and PSD for example.
There was an app called Pixie in the early days of KDE, perhaps you can find something there. Otherwise, I'd suggest looking at what the GIMP does.
Well, here you have a list of all the formats QT 4.6 QImage support:
BMP Windows Bitmap Read/write
GIF Graphic Interchange Format (optional) Read
JPG Joint Photographic Experts Group Read/write
JPEG Joint Photographic Experts Group Read/write
PNG Portable Network Graphics Read/write PBM
Portable Bitmap Read
PGM Portable Graymap Read
PPM Portable Pixmap Read/write
TIFF Tagged Image File Format Read/write
XBM X11 Bitmap Read/write
XPM X11 Pixmap Read/write
And here you can see all the images available everywhere. Tons of them. Like a hundred:
Wikipedia. Comparison of Graphics File Formats
And the popular:
JPEG · JPEG 2000 · JPEG XR · lossless JPEG · JBIG · JBIG2 · PNG · WBMP
APNG · BMP · DjVu · EXR · GIF · ICER · ILBM · MNG · PCX · PGF · TGA · TIFF
Here's a plugin which uses the DevIL image library to support a variety of formats.
The page is in French, but the example code is straightforward, and Google Translate seems to do a good job with the text.