I think you sould follow the next important steps

1. Make a backup of your files and database (finally)

2. Make sure you have set all the files and folder permission right as suggested to the following thread

https://www.zen-cart.com/tutorials/index.php?article=9

3. Test your store with the default or classic template to make sure that the problem is related to the template you are using.

4. If you want to unistall the IH3 follow the instructions inreadme file as follows.

"In Admin > Tools >Image Handler3, click on "Remove Image Handler from database", then delete all Image Handler3files. (be sure to replace core Zen Cart files with the default Zen Cart versions of these files. You will need to make sure that any modifications required for other add-ons must be included in these replacement files.)"

5. After you ensure everything is back to normal....BACKUP EVERYTHING AND....

6. Start over :)