As locking large images takes a lot of time I started to search for some optimizations.

The best solution would be just to split image into parts.
And here's the problem:
There is always visible line between parts, very visible if edge area is not too colorful.
It looks to me like few pixels from the right side are displayed on the very left.
Boxes look like every sides texture just start on previous side.
Creating plane or two triangles changes nothing.

Is this something that can be overcome?

That's probably related to texture addressing, Clamp should be used instead of Wrap for this case
VI.clamp

I'm normally using mesh (.parts.New() .base.createPlane(2, 2, VTX_TEX0 | TX_NRM)), but I checked VI now and can't see any difference between clamp & wrap for this case.

If you're using mesh rendering for images then it will always use wrap tex coords.
So you might want to draw the images manually and not using meshes.

I added transparent borders and wrapping seems to be no longer a problem, but there is still a line between images like they are blended in some strange manner instead just draw one next to another.

But nvm, I'll split images manually, which is in fact better except of being boring.