First of all, you must have the Gimp-Print drivers for your printer installed. Then using Portraits & Prints from econtechnologies and their Template Maker software, create a template for 4x6 paper and make a photo box to fill up the entire template. Import this template into Portraits & Prints and simply choose this as the layout when printing.
Borderless roll paper printing has been working fine for me and the output is great. The only problems I have noticed come when printing a lot of pictures at the same time (23 of them). Up to nine pictures at once seems to work fine. Also, after one print job has started, do not submit another one because the first one will mysteriously stop.
[Editor's note: I have not tried this one myself...]

