as you have found out, the "generic" control is generic. It uses the most common constructor of window classes in wxWidgets but they do not fit all needs.
The code compiles but the image does not show in the sizer
From here on it is no codeblocks problem anymore, and you will need to search for help in a wx forum. We can not support this kind of questions.
const wxString Filename("Images//TIA Cascode & Bootstrap cropped.sch.png");
const wxBitmap SchematicCascodeBoostBitmap( Filename, wxBITMAP_TYPE_PNG );
you do not make any sanity check if the bitmap is loaded. This seems to be a bad idea for me. Here it can fail.
You can try to debug...
The image displayed, but not inside the sizer
So the image gets displayed but is not inside the sizer, or does not scale properly? Have you checked if the generated code is correct? Is the custom control added to your sizer? Have you set the expand flag? This is all wild guessing without code, image or video...