Yeah, an image file can't delete itself, but there are quite a range of ways they could get lost, ranging from an inadvertent finger press through hostile action through to hardware failure.
If they are on a removable sd card, where did you buy it from? A long-running scam is hacking a smaller card to tell the device you put it in that it's larger than it is, then selling it as a high-capacity card. When you exceed the real capacity of the card the phone doesn't know it's full and so will overwrite existing data. Cards bought through under-regulated internet sites (eBay sellers, Chinese online markets, etc) are a high risk of this, but they occasionally make it into the supply chains of more reliable sources.
