Adding SDL2_image with JPEG support for iOS is a little bit different than for Android. In case of Android, it was necessary to add JPEG library in C and build it. iOS has JPEG dependency hidden in another library which is already compiled in frameworks.
If you just add SDL2_image to your project for iOS, you will very likely end up with following linker error:
To resolve this issue, it is sufficient to add two dependencies into your project.
Go to Project and select Build Phases.
In the section Linking add library: ImageIO.framework
Then add the second dependency: MobileCoreServices.framework
Then Clean and Build the project.
These steps should resolve the linker issue, and JPEG should work.