Hi, anaiarasantos,
If it is a glitch caused by the update to iMovie 10.1.15 then Apple may be the only one that can cure it.
The recommended way to import photos into iMovie is to first place them in the Photos app, and then access them in iMovie form the Photos item in the media sidebar. Have you tried that?
If still no luck, try working through the following diagnostice procedures to see if one of them will cure it:
Deleting Preferences
Open iMovie while holding down the Option and Command keys and select to delete
preferences in the box that appears. iMovie will open in a new library. Reopen
your old library to get back to your projects.
Opening iMovie in a new library
Try opening iMovie in a new library. To do
that, open iMovie while holding down the Option key and then selecting to
create a new library in the box that appears.
Opening iMovie in a new user account
Create a new user account from System Preferences/Users and Groups and try to open and
use iMovie in the new account.
New Project
Another thing you can try is to copy your media into a newly created project and see if
that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the
media in your project and then Edit/Paste it into the newly created project.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and,
after everything loads, immediately shut down and reboot in normal mode. Then
see if iMovie works properly.
Updating iMovie
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications
folder into the trash, but do not empty the trash. (You can skip that step if
you had iMovie 10 on your computer before, but deleted it.) Sign in to the app
store and open your purchases folder by clicking on your name at the bottom of
the app store side bar. (NOTE: For El Capitan and older, click on the Purchases
tab at the top of the app store screen and then sign in. Click on Purchases
again to reveal your purchases.) You
will see your purchases displayed, including iMovie 10. Redownload/update
iMovie from there. If not successful, drag your present iMovie app out of the
trash (if you moved it there) and return it to the Applications folder.
Resetting the System Management
Controller
Try resetting the System Management Controller:
https://support.apple.com/en-us/HT201295
Resetting the NVRAM
or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
-- Rich