Long story short: To make a header look nice on both mobile and desktop, do the following:

1. Upload an image that is 625x352 pixels as your header via the “edit theme” button. This is your Mobile Header.
2. Upload another image that is 1280x300 pixels somewhere else. You can use an image hosting site, or you can make a private image post on your Tumblr. I suggest the “private Tumblr post” option, because then you don’t have to rely on a separate web site to keep your image uploaded for as long as you have your Tumblr. This is your Desktop Header
3. Get that image URL. When I say this, I do not mean the URL of the post, I mean the URL of the image itself. If you do not know how to do this, then click here.
4. This step is optional, but I recommend that you open a new tab (or window) and paste the URL you just got into the address bar to view your image. This way, if you accidentally copy something else before you finish, you can just copy it from the address bar of that tab instead of redoing the previous step.
5. Go to your Tumblr page, select “edit theme,” and then click “Edit HTML.”
6. Search for “.header-image.cover”. You should eventually see this:
   
   .header-image.cover {
                  background-image: url({HeaderImage});
   
   
7. Replace the “{HeaderImage}” with the image URL you got in step three.
8. Hit “Update Preview,” and then “Save.”
9. You’re done. Yay!

Just a note: This was the result of a lot of experimentation on my part. I’m not an expert on Optica. I just decided that it was such a hassle to figure out that I might as well make things easier for people with the same problem who probably don’t have the experience needed to figure it out on their own.

Also, I’m not completely sure this method works for all computer screen resolutions. I tested a lot of them, but not all of them.

If you have any questions, feel free to shoot me an ask. If you send the ask and I don’t get to it relatively quickly, feel free to send another one. If I reply with an “I saw this, and I’ll get back to you when I can,” and then I say nothing for a couple days, then also feel free to send another ask. I don’t have the best memory, and I sometimes need that reminder.

Please do not message me if you want a prompt reply. Send an ask or use the submit link. I tend to use the mobile web site (not the app) a lot more than the desktop web site, and although I can check my ask/submission inbox on mobile, I can’t see messages at all on my phone.