Pictures lost original size and resolution.?
Not sure which “internal memory” you’re referring to as you failed to mention the device. But your only hope is if the original data is still in the internal memory. Even if the device claims the internal memory is now free, the data will remain unless it has been totally overwritten by new data. Good file recovery software can recover that and the best will also do “raw recovery” which will even recover after reformatting the internal memory. There are a lot of data recovery apps and software with different strengths and weaknesses, but the best ones are certainly not free. If you have already been reusing the internal memory extensively then there will be nothing worthwhile left to recover. The only certainty is that if the data on the SD card has been resampled at lower file size and image detail and that’s the only data you have left, then there is nothing which can recreate the data which got dumped during the file conversion.
Do yourself a huge favor and set your device (you should have stated what it is) so that it NEVER uses internal memory ever again. SD cards are dirt cheap, so there's absolutely zero reason why you should ever need to use internal memory. Furthermore, simply using internal memory should not have lowered or altered the image in any way unless there's some kind of predetermined set of editing that the device *(whatever it may be) is preprogrammed to perform when a file is saved to internal memory.
Take out the memory card and check the camera if it still has the pictures in its internal memory. If the pictures are still there and looking good, connect the camera (without the memory card) to the computer via a USB cable. Use the computer's file manager to copy the pictures from the camera's internal memory to the computer's hard disk. If the pictures come out bad, try another USB cable. If the pictures appear bad in the camera then there's nothing you can do with them anymore.