If that's the case, you can either:
a. Use the mobile version of MUT by logging in on your phone and clicking the "Mobile" button instead of Desktop on the very bottom of the site. Then, find this thread and use the "reply" box to upload your picture. Click on the little house again (3rd button) and choose "upload from my device" and follow the steps (this works when I use my iPad or iPhone, but I can't speak to other devices). Then, to make it a spoiler, type [SPOILER] at the beginning of the post and [/SPOILER] at the end - since there is no spoiler button on the mobile version of the site, you have to add it manually.
or b. Email the picture to yourself from your phone, download the image, then upload it to your post from your computer as explained above.
Let me know if you have any questions! I know there are a few steps but the process is easy :)
^^^Awesome! I didn't realize that you could do a spoiler from your phone. Most excellent...thank you!