Printing .png images - getting lines around image bounding box

[TheRobsterUK]

I've been using SCAL5 Pro in conjunction with my Skycut vinyl cutter to make stickers. In general I design the stickers in Inkscape and then export as .png (at 600dpi) and then import these into SCAL for Printing & Cutting. However I have an issue that when I print (to my Canon Pixma Pro 100s) sometimes some some of the images are printing with a black outline, usually along two sides of the bounding box of the image (see attached file).

I've been told this happens to other people as well, not necessarily using SCAL or the same set-up (not the same printer etc) so it's possibly a bigger issue than a bug with SCAL. Just wondering if anyone on here has had a similar issue, knows what causes it, how to prevent it etc.

(note that in the attached images, both are the same imported image, the top one is just a mirror of the bottom one). However the bottom one does not have the lines around the two edges when printed.