With care and skill, it is certainly possible to successfully shrink JPGs.
Simple, basic, logos are easiest of all.
This kind of shrunken reproduction is very commonly done by people with the necessary desire, skills and equipment. Whether it is worth the personal effort in developing the skill is debatable for only the odd piece of work - definitely not for me, but each to his or her own
The usual pitfall associated with JPGs is that it is a
lossy compression format. The problem lies with saving, reloading and re-editing the saved picture, time after time, with loss of quality on every subsequent generation, just like photocopies of photo copies. The 'trick' is obviously NOT to keep resaving/reloading, but to do ALL the required work and then save just once. The other common sense precaution is to ensure that any saves are done with the least possible compression and maximum file size - who cares how big the file is as long as the end product is acceptable!
Another approach is to convert the original file to a lossless format before doing any work on it, but it simply isn't necessary for anything as simple as the average basic logo.
Final result will obviously depend on several things - the quality of the original image, how much detail is present in the original and how much care and skill is employed in the resizing and in touching up that might be necessary. After that, reproduction will depend on the quality of the printer and at least as much on the medium that the image is finally printed on. Reproducing white depends absolutely on the white of the paper, card or decal film, unless you have access to an Alps printer or similar that can actually print white. Similar applies to metallic colours.
If the original JPGs are good, then it should not be a massive problem, using almost any half decent image editor. If the originals are not good, then the reduced size copy will naturally reflect that, but even so, can still be retouched. Careful pixel editing can restore detail to almost anything, as long as it is a relatively basic, simple image, which logos almost always, are.
A common problem may be found in the printer inks not reproducing colours accurately enough, but, as already mentioned, that can be at least as much the fault of the receiving medium as the ink itself. Printers, paper/film and inks can vary considerably. You can spend a good deal of money on inks and print media while experimenting until you develop the necessary skill and experience.
The problem with something like a tiny loco name plate is not in reducing it to size, but in obtaining a suitably high quality original image of such a small object in the first place. If a suitable high quality scan can be made of the original object, in this case it wouldn't even need reducing in size.
