QUOTE (StuBeeDoo @ 21 Jan 2012, 14:55) <{POST_SNAPBACK}>Thanks everyone.
I'm in the (slow) process of finding my way around Inkscape.
Before you get too hung up in the software...
I notice you said by the time you get the image small enough you lose definition. This implies the original jpg's were adequate seeing as bitmaps are only a problem if you make them BIGGER not smaller.
Printers print in dots (like jpg's) and not lines (like vector images). So a printer will convert a line drawing into dots in order to print it.
It seems to me that your problem is more likely the PRINTER RESOLUTION rather than the images.
When you print you need to set the highest quality setting your printer has. What you print ONTO also makes a BIG difference - some media "bleeds" (the ink spreads out and blurs slightly). This is very noticeable when printing small and expecting detail. In my experience some of the decal papers are very poor at high resolutions so try a more expensive and better quality decal paper.
If your decals need a white background and you aren't too worried about slightly thicker decals you could try buying some printable vinyl (only suitable for inkjets). I use this to make decals for my transit vans and it's great for detail and it's self-adhesive but not so sticky you can't peel off a cock-up! I tend to seal it after with clear acrylic spray. Search for printable vinyl on ebay, that's where I got mine.
The vinyl will be great for rally plates and number plates. It won't be so good for complex decals that bend over compound curves (unless you are really handy with a craft knife).

I'm in the (slow) process of finding my way around Inkscape.

Before you get too hung up in the software...
I notice you said by the time you get the image small enough you lose definition. This implies the original jpg's were adequate seeing as bitmaps are only a problem if you make them BIGGER not smaller.
Printers print in dots (like jpg's) and not lines (like vector images). So a printer will convert a line drawing into dots in order to print it.
It seems to me that your problem is more likely the PRINTER RESOLUTION rather than the images.
When you print you need to set the highest quality setting your printer has. What you print ONTO also makes a BIG difference - some media "bleeds" (the ink spreads out and blurs slightly). This is very noticeable when printing small and expecting detail. In my experience some of the decal papers are very poor at high resolutions so try a more expensive and better quality decal paper.
If your decals need a white background and you aren't too worried about slightly thicker decals you could try buying some printable vinyl (only suitable for inkjets). I use this to make decals for my transit vans and it's great for detail and it's self-adhesive but not so sticky you can't peel off a cock-up! I tend to seal it after with clear acrylic spray. Search for printable vinyl on ebay, that's where I got mine.
The vinyl will be great for rally plates and number plates. It won't be so good for complex decals that bend over compound curves (unless you are really handy with a craft knife).