GIMP is an acronym for Gnu Image Manipulation Program, a free image
tool that runs on Windows, MacOS, Linux and other operating systems.
It's best to download it from the GIMP download page:
https://www.gimp.org/downloads/GIMP v2.8 is almost 90 megabytes, so it will take a while on a slow or flaky connection.
When In Danger, When In Doubt: Do a web search for
'gimp Whatever You Want To Do'
This should give you a choice between GIMP's own documentation, GIMP
users blogging and possibly 'how to' videos.
Navigate to the image you're starting with. GIMP can read most types of images, but may be slow handling files larger than 100 megabytes.
Make the resolution suitable for printing in your scale. Images found on the Internet or scanned from paper may have resolutions between 72 Dots Per Inch and 1200 DPI. Your prints may be pixelated and jaggy if the resolution is less than 300 DPI (each dot is about 1/4 of an HO scale inch).
Set 'Interpolation' to 'none' and change only the 'Resolution' value. HO scale billboards and business signs look good printed at 600 DPI. HO license plates, window signs etc. need 1200 DPI if you want to be able to read them under magnification. 1200 DPI files are much bigger than 600 DPI. Not all printers can do 1200 DPI.
Only if the part you want is rotated relative to the overall image. View -> Show Grid will put guidelines on your display.
GIMP offers a lot of selection tools. 'Rectangle select' is simplest and will handle most signs. Other tools select by color or let you click a succession of points on the boundary of an irregular object.
Often the camera's film or sensor wasn't square to the part of the image you want to use. This tool lets you 'tug on the corners' to fix perspective problems and distortion from scanning a page from a book.
You may need to Crop again once the perspective is fixed. Remove anything you don't want on the final printout.
If the image shows physical damage, flash glare or discoloration, pick an intact spot of the right color and paint it over the incorrect areas. The brush size is settable down to one pixel square. The shape and edge sharpness is also settable.
Like 'Clone', except the source area moves as you move the painting brush. Used to replicate a color gradient or pattern.
Check dimensions of the cropped and corrected image. I set the units to 'millimeters' to take advantage of HO being 3.5 mm per scale foot.
Set the final size of the printout. I set the units to 'pixels' to get the best resolution, switching to 'millimeters' to check vs. the desired HO scale dimensions.
GIMP uses .xcf files to store images. You can print from GIMP, but you may find your printer can't do the resolution you want, or that inkjet inks don't hold color over time. I've been printing my images to a PDF file and sending that to a nearby print shop which has a color laser printer.
Floating Selections are a GIMP feature I haven't found much use for yet. Clicking elsewhere in the image will unfloat the current selection and let you do whatever you want to do next.
GIMP supports putting pieces of an image into different layers so you can work on each individually. But this can make it look like the program is stuck - the tool you're using doesn't do what you expect. Image -> Flatten Image will get you back to a single layer.
Unless you're working on a backdrop, you'll want to put more than one image on a single page. Start with one of the ready-to-print images you made with steps 1-10 above.
Set the dimensions of the canvas to a little bit smaller than the print area (paper size less margins) of the printer you're using.
When you've got everything you want added to the target image, save it and print as discussed above.
Note that really complex, high resolution print files may be too big to email. Most print shops let you bring in a thumb drive. Or just split it into two or more pages.