Upload pictures

Anyone can tell me where are the functions for uploading and resize images in the oxid framework 4.5.1?

Uploading and resizing are two separate processes, there is not relation between them. Resizing is done on the fly depending on theme settings.
However, uploading is done in oxUtilsFile->processFiles()

In case you have encrypted OXID edition, you simply can get the open source community edition of the same shop version and inspect it’s code.