Which formats work
JPG, PNG, WebP, BMP, GIF and AVIF all decode directly. HEIC photos from an iPhone depend on the browser: Safari on a recent iOS can usually open them, most other browsers cannot. If yours cannot, OCR2text says so plainly and suggests exporting a JPEG rather than silently failing.
Very large images are scaled to a sensible recognition resolution first. Beyond a certain point extra megapixels stop helping the recogniser and start costing memory, so a 48-megapixel phone photo is resized rather than pushed through whole.