The image I uploaded was rejected because it was under 300 DPI.
The DPI is just a number which has no relevance to a digital image. It relates only to printing and not the quality or resolution of the image itself.
If you change the number to 300 DPI in an editor, it should be accepted.
As you have Windows 10 and presumably use assistive technology to e.g. read screen dialogs, I suggest trying the following.
Install GIMP which is a free image editing application. GIMP is available at this webpage. I use GIMP myself.
Open your image in GIMP.
Now go to the menu option Image > Print Size. This will open a dialog box that let you set the DPI. There are two fields in the dialog labeled "X resolution" and "Y resolution" - you can ignore the other fields. Change the "X resolution" to 300 and the "Y resolution" will change automatically when you press TAB. Once done, press the OK button.
Now go to the menu option File > Export As and you can save the file under a new name. That saved file will have the DPI fields set at 300 DPI.
This works on my Linux setup, and should work the same on Windows 10.
I have tried to make the instructions as clear as possible given that you are blind, but get back to me (using comments) if there is a problem and I'll try and help.
As the OP asked for a command line option in Linux I can tell you the simplest way is to use ImageMagick.
You need to do this :
- convert input.jpg -units PixelsPerInch -density 300 output.jpg
With the filenames changed of course.
Note the "-units PixelsPerInch" option. This is absolutely essential or the file will be marked with an unidentified unit type and not render properly in print applications.
Note that I checked this very carefully using my own Linux setup and checked output rendered by Latex and the PixelsPerInch part is essential to get valid rendering. People sometimes leave this out and it seems to be a source of problems when they actually print the image.