It depends on how you lost them and where there were.
If they were on a removable sd card, or any storage device which can be mounted as USB mass storage on a computer (i.e. appears like a usb flash drive) then you just use a file recovery tool to retrieve them. If you are using Windows then many people recommend Recuva, which is free. The key is not to use the device in the meanwhile, since any new data saved may be overwriting the deleted images.
If however they were on a device which can't be mounted this way (usually the case for internal storage on a phone) you are in trouble. If it's using "MTP" (Microsoft's Media Transfer Protocol, the default in recent androids) then the file recovery tool won't be able to work through that interface.
So that's really the key: can you mount the storage device like a USB flash drive or not?