new photo has corrupted my iphoto albums

What can I do to fix the thumbnails? It might seem silly, but they're kind of important!

See this User Tip: How to Rebuild Thumbnails in a Photos Library

Use this simple workaround to fix the thumbnails:

• To rebuild the thumbnails edit them slightly, so Photos will be forced to render new thumbnails. This is best done by rotating the photos back and forth, because rotating by 90° degrees is non-destructive and can be applied to multiple photos at once without having to open the Edit window for each photo individually.
• Select all photos that need new thumbnails at once - it does not matter if the selection includes photos that do not need repairing.
• Then rotate the selected photos back and forth by first pressing ⌘R (Image > Rotate Counterclockwise) and then back by ⇧⌘R (Image > Rotate Clockwise).

This may not suffice, if your photos are RAW photos and the corrupted thumbnails are caused by a difference in RAW processing. In that case try to reprocess the RAW files:

Double-click a photo in your Photos library to open it, click Edit in the toolbar, then choose Image > Reprocess RAW.

Before applying this fix I'd repair the Photos library, just in case there are other hidden corruptions.To repair the Photos Library:

• Make a backup copy of the iPhoto Library.
• Hold down the key combination alt/options key and command key ⌥⌘ while double clicking the Photos icon. Click the "Repair" button and repair the library. This may take a while for a large library.

As to your second question:

Also, HOW can I delete photos from my camera roll after importing them to my computer? It's driving me crazy. One thread said they can no longer be deleted, but I can't believe that's true- I can't believe that I will be stuck with every photo (especially the garbage ones) cluttering up my phone forever.

Thank you in advance for any help or suggestions the community can offer and Apple, would you please fix Photos?

You still can delete the photos from the Camera roll after importing the photos, but it will only work, if you do not enable iCloud Photo Library on your iPhone. Are you using iCloud Photo Library?

Select your iPhone in the Sidebar and click the "Delete items after import" button, if you want to delete the photos.

Right now, when I delete a photo the dialogue box says it will be deleted from "all my devices".

That is what iCloud Photo Library is doing. The photos are stored in iCloud and only mirrored on your iPhone. If you delete a photo on your iPhone, it will be deleted from iCloud, and thus from all devices that are syncing iCloud.

After you disable iCloud Photo Library on your iPhone, you will see your Camera Roll again and can delete the photos from the Camera Roll, without deleting them from the other devices that are syncing with iCloud Photo Library. iCloud Photo Library FAQ

You're welcome 🙂

Why can't I backup my photos on iCloud and also delete them off my phone. I do not want every photo I have ever taken on my phone, I need the space. Once the photos are on my computer and iCloud I do not want them on my phone anymore. What am I missing?

Because it is not possible to backup photos up to iCloud - that is not a feature

and if you are using iCloud Photo library then any change made to the library from any device is reflected on all devices

If you have a space issue on a device then set that device to use optimized photos with ICPL

LN

PS: And it is Rude to thread jack other people's threads - in the future please start your own