Some websites prefer photographs to be in a 3:2 ratio to match their layout. If you upload a floor plan image with square dimensions, then the image may be automatically cropped and it might cut out some of the floor plan.
To prevent your floorplan from being cropped, you may have to change the image's aspect ratio to match the 3:2 standard before you upload.
How to Change
You can use any image editing application you want, as long as you can resize and crop the image. The following steps use Skitch from Evernote.
Launch Skitch and go to File > Open. Open the floorplan image (PNG file) to edit.
Go to Tools > Crop/Resize to open the crop tool.
At the top you'll see the current resolution. In the image below the dimensions are 685 x 606. (This is different from the actual resolution in the lower-right corner).
Usually the image will be square so you'll want to make it wider to fit the 3:2 ratio.
First calculate the target width based on the current height.
|TargetWidth = 1.5 * CurrentHeight|
In the image above, the current height is 606 so the target width will be 909.
Move the cursor to the left edge until it changes to a new icon. Click and drag out to increase the image width. Keep the floor plan centered by expanding to the left and to the right. It's okay if you're off a few pixels from the target width.
Click Apply to save your edits. Then go to File > Export to save the new file to your computer.