Some colleges here in the UK use Gimp as it is like Photoshop but free to use. Lots of Gimp Tutorials exist but this one may help you to get an idea, skip to 2.07 to pass the boring bit Gimp v Photoshop etc.
https://www.youtube.com/watch?v=_L_MMU22bAw
Connect all tool windows into one:On the File Menu go to "Windows" and choose "Single Window Mode".
Construct an oval picture:Open pic
Click on OVAL in left menu
Click on pic and drag as big an oval as you can, book ends extend the size horizontally and vertically
File Menu choose "Select" then "Invert"
Hit "DEL" on keyboard
Save it by Exporting it which is "Export As", which is the same as, "Save As", ok on sizes
Change to .PNG if necessary
Rename to SampleOval instead of Sample or it will overwrite the original image
Original pic = Sample.PNG
Exported as SampleOval.PNG
It is possible to make a secondary oval slightly larger or smaller than the first then colour it to make an oval frame.
It also works with layers and lots of tutorials existNOTE: My nick and image are from a company. That company allows people to use them both as long as they get permission and a licence. I have that and a certificate to prove it.
"These are not the Druids you are looking for..."