Easy-to-use image-editing FLOSS

GIMP and Inkscape aren't comparable: GIMP is a bitmap editor, Inkscape is a vector graphics editor.

GIMP is the uncontested leader amongst open source bitmap editors, feature-wise. It ranks closely with Photoshop. However GIMP has poor support for the CMYK color space, which is used by most “serious” color printers; I'm told this makes it unsuitable for many applications. As usual Wikipedia has a comparison of raster graphics editors, which may or may not include all important criteria.

There are many different types of vector graphics. Inkscape is a leader in its field, which is two-dimensional general-purpose schemas and scalable graphics, made mainly of straight lines, simple curves and filled regions. Other programs may perform better for CAD, electronics schemas, geometrical constructions, state diagrams, fonts, 3D modeling, etc.

The GIMP is definitely the go-to image editor on Linux. It has the best psd support of any native editor or viewer. You say "it has too many problems" but you don't explain what any of those are. It is quite stable, actively keeps up with the latest developments in the OS and implements bug fixes in a timely manner. Since you seem willing to consider Photoshop and even things that run in Wine, complexity and size don't seem to be your concern.

Allow me to suggest that The GIMP can probably be customized for your purposes. The default interface can be configured to show very little besides the image window and one toolbar (say with just the pan and zoom tools) which can have the layers dbx attached for simple viewing operations.

The most likely thing that comes to mind is that you might consider a "problem" is how different it is from Photoshop. It does approach things from a little bit different angle. If these are your problem you could consider some of the config file / plugin combinations that make some of it's behavior more recognizable, although I actually suggest you will be better off in the long run learning a little bout how it works for it's own sake.

If you more explicitly state what problems you see in it, you might get help fixing those. This is likely to be less work, more stable, and better integrated with the rest of your Linux environment than a foreign piece of software under Wine.