Go to https://steamosaic.com/ and put in your steam ID, hit enter and wait for it to bring you to your image (your games need to be public for this site to work)

Go to https://pinetools.com/split-image and upload your image, on the options section click Both (grid) and set it to be however your showcase is (Horizontally 5 and Vertically either 1, 2, or 3 depending on workshop showcase level) then hit split image and download each image

I suggest having Steam Artwork Hub Essentials on chrome/any chromium browser https://chrome.google.com/webstore/detail/steam-artwork-hub-essenti/kdappijekicdbempkjaahnnmkookhfnj then go to https://gtm.steamproxy.vip/sharedfiles/edititem/767/3/ and drag and drop all images that you prepared and click "Hide Title", "Enable Transparency, Long Workshops and Long Guides", and set the What are you uploading to Workshop make sure to keep this page open after clicking upload at the bottom of the page

For this I'd suggest having both pages open in separate windows so that you can more easily match up each file to one another as you cannot tell by how it looks, in the upload an artwork page hover over the File uploaded successfully and you will see it links to it with a specific ID, head over to your featured showcase workshop and hover over each one looking for the same ID and click it, I suggest working from bottom to top because it will make it easier to select each row's images after you finish one. Click save at the bottom of the profile showcases page and voila you are done, now you can ogle over your new masterpiece on your profile!