How to webClient.write() an image stored on a String?
You should not use String to store binary data.
Instead allocate yourself an array of uint8_t of the right size.
uint8_t *imageData;
size_t len = file.size();

imageData = alloca(len);
file.readBytes(imageData, len);

webClient.write(imageData, len);

With that said, without knowing what the rest of your program is doing I can only surmise that you're just blasting raw data at the web server and hoping it sticks. That's not how web data transfers work. You have to craft the upload page properly as multi-part mime form data. Read this for more information