The easiest on-ramp to speeding up image/video processing is probably the npp library https://developer.nvidia.com/npp [nvidia.com] It has functionality and syntax similar to Intel's ipp library but uses an NVIDIA cuda-capable GPU to accelerate the operations.
If you want to dig in deeper you could explore OpenACC http://www.openacc-standard.org/ [openacc-standard.org] OpenACC is a directives based approach to accelerator programming. You comment or mark up your code with OpenACC directives that provide additional information that the compiler can use to generate parallel code.
Finally, you can learn CUDA C, or OpenCL, or CUDA Fortran, or NumbaPro, or one of the other programming languages that are supported on the GPU hardware of your choice. NVIDIA's CUDA C compiler is based on LLVM and the IR changes have been upstreamed to LLVM.org, There are several languages and projects in development that are leveraging the LLVM infrastructure to add GPU/parallel support.
[disclaimer: I work for NVIDIA, but the words above are my own.]