The key is indeed using higher resolutions. I use photoshop. I make a canvas that is sized to printer paper dimensions, 8"x11", then select an area that's sized to the SuperCal decal sheet I use, and place it in the upper left corner. I also have the ruler tool enabled so I can keep things in scale to the model. When you reduce the image, you want it reduced in relation to a full sheet of paper and not the smaller decal sheet, which is why you make your canvas to the larger of the two sheets
Sounds like you're working in 1/35th scale or 54mm, so you may be out of luck regardless. It's awfly small for the resolution of an Inkjet, but keep trying. You should be able to get divisional patches to print okay, though they may not be too clear, but they will be recognizable. You are likely out of luck with the Death's Head badge though. I fear that it's too small. If you can't find images on the web, don't forget your scanner and your reference library.
Here's a pic of some decals I recently made for a model of a local county sheriff's department car from about 1995. I did these on my Inkjet.
Not the clearest of pics, but you can clearly read the writing on the badge aswell as see the image in the center of the badge.
Print off as many tests as you need to to check the scale, visibility/clarity, and position on the page before committing the image to decal sheet..