Ink vs Laser isn't important as it's no different here in result. You just need to purchase paper appropriate to your actual device. I suspect you were reading me as stating there's a difference between their results as opposed what I'm writing is that paper for one will not work on the other. My home printer is an inkjet incidentally.
There's a plethora of brands out there. I use the Testors stuff myself though Tamiya and Hasegawa make products well regarded as well as I understand it.
Crafting images to be print in decal paper isn't really much different that on a regular sheet of paper. Or I should say, at the level that we're aiming for differences are so minuscule as to not even matter to the non-professional. I'm working on the assumption that basic print and graphic knowledge is here. All the "normal" stuff about DPI and calibrations are in play just like you're trying to do a nice print on your own paper (basically just like doing a nice digital photo print). Don't try, especially at 6mm, to get super detailed with gradients and too large; You're probably not going to have a good time and at that point, it's just better to by professional crap because it will cost you less in time and money in the long run. There's a RIO that only you can determine but even at FW prices-especially considering that you can get knockoffs for CHEAP on ebay (and no I can't link direct to any of them using the forum SW as that's against the rules) for a limited run, it may just be better to get pieced out sets. YMMV.
In totally unrelated topic, I hear the xyzmall-china store is nice for modelers. I've not shopped there personally of course.
If you can work it, a vector based format
may work better for you but again, when you're printing something 4mm wide, a raster image with enough DPI will work just fine. The only limitation is your own printer. What's good about that is you can figure this out on cheap regular paper just fine printing say at 300DPI the same image at progressively smaller sizes in a pattern on the test page and see where you fall off into not being acceptable.
While I can go hog wild with gimp or photoshop on this, I've never needed more than the free Paint.Net tool as everything else is monster overkill. Meaning you can easily get away with just about any tool, something fancy is not really required. You're making something under a centimeter and often far smaller. You're tricking your eyes into seeing the thing you want to see, not really needing oodles of details (again YMMV).
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