In a standard table cell, cell.image specifies the image. Is there a preferred size for the x1, x2, and x3 images? I tried square pngs of 25, 50, and 75 pixels on a side and this looks fine in the simulator.
Yes, 25, 50 and 75 px are the standard size for icons (both cell.image and toolbutton images) on iOS.
When Xojo will support Android, the standard size is a multiple of 24px
If you are looking for icons, I would recommend http://icons8.com