DSLRs and compacts shoot images in a different frame ratio to video which is either 16:9 HD or 4:3 for standard definition.
DSLR still are approx 5:4 ratio.
In order to have your stills 'fit' the video frame correctly without stretching, squishing or cropping the imported image must be at that frame ratio.
If you wish your still ratio not to be altered in any way then you will have dark bars at either side.
If you want your image to fill the frame then you will have to decide on a crop. Top or sides
A couple of ways In Photoshop, I guess there must be an equivalents in PaintShop, goes like this.
First quick and dirty:
export a still from your FCPX timeline.
Open in Photoshop.
Select Image - Image Size - note the dimensions - 1920x1080@72
Change the Resolution to 150 or 300 if you are going to zoom your image in FCPX.
Close window.
Select Crop tool - set the values to Front Image.
Your crop tool can now be used to crop your images to 16:9. @150/300
the second way if you want to create graphics or work on your images first.
Select File - New -
In the dialog box drop down select Film and Video
Choose HDTV 1080p.
Notice the frame size and pixel dimensions - 1920x1080@72p/i.
This will form the canvas in which you can create or drag and drop your image into.
Your stills will certainly be larger than 72dpi resolution so you have to keep resizing in order to get the cropping you want.
Once finished export as jpg or png.
Note:
you will now have an image that will fit exactly ..however if you wish to zoom make sure you have set your resolution to 150dp //300dpi. This will allow you to perform x2 x4 zooms without pixelation in FCPX.
BTW I use Lightroom which blows Photoshop or PaintShop out of the water.
If you intend to import/process more than two or three images into FCPX then its a no brainer.
If fact I stopped using PSD for stills years ago, can never go back.
I can batch crop, color corrected, filter, effects etc..export to FCPX 1000 timelapse stiils in around 10 minutes.