NOTE:
Regardless of whether you made your BP in Sandbox, Franchise or any other game mode it is always best to take screenshots in Sandbox mode as you can easily adjust the lighting/time of day.

TAKE YOUR SCREENSHOTS
Make sure to press 'G' to hide the UI and ensure your item is decently lit, once you have lined up your shot press the 'prt sc' button on your keyboard to take a screenshot, this button is to the right of F12 on standard UK keyboards but may vary with other builds so you might have to search for your particular keyboard set up to find the equivalent button.

SAVING YOUR SCREENSHOT
Open the 'Windows Paint' app and click on the PASTE icon or press CTRL + V
You should now be able to see the screenshot you just took and you need to save it so click on FILE at the top left of the app and in the drop down menu navigate to and mouse over SAVE AS and in the new drop down menu select JPEG picture (do not use any other format as these create files too big to be used by the workshop)
In the File Explorer window that pops up navigate to the folder you want to use to save your screenshots and re-name the file from 'untitled' to something unique and click on SAVE (remember the location of this folder, you will need it later)
Once saved go back to PAINT and click on FILE again, this time select New from the drop down menu ready for your next shot to be loaded.
Return to PZ and change your lighting or position ready for the next shot, repeat the process above to take and save screenshots until you have enough to properly showcase your item.

Navigate to your workshop item and on the right hand side select Add/edit images & videos
STEP1
In the workshop click on Choose files and a new window will pop up in File explorer
STEP 2
In File explorer navigate to the folder where you saved your screenshots (after the first use this location should now open automatically in the future)
STEP 3
Click on the shot you want to upload
STEP 4
Click on OPEN
STEP 5
The workshop window will now re-open and the name of your screenshot will appear in the box next to the 'choose files' button
STEP 6
Click on 'UPLOAD' and after a few seconds your screenshot will appear at the bottom of the screen
REPEAT steps 1 to 6 until you have uploaded all the screenshots you want to use
STEP 7
Finally click on SAVE to finish and return to the workshop view
ADDITIONAL NOTES
While in the screenshot edit window you can delete unwanted shots (hover your mouse over the screenshot and click on the red 'X') or move them to a different position by dragging them to a new location in the sequence. If you want to replace a shot with a better one just take a new screenshot and save over the old shot in PAINT then re-upload it to the workshop, it will overwrite the original shot. Do not delete screenshots you have uploaded from your File explorer folder or they will no longer be available to your workshop item.

If you found this guide useful please give it a THUMBS UP as it helps make it more prominent and easier for others to find and I hope this helps you make your workshop items more appealing.