'dd' has overwritten your partition for the length of the '.img' file. All content beyond that length is still valid, but not accessible. You will need to set up all meta-information (nodes, etc) again, somehow masking out the over-written part. This needs an amount of skills and time that is usually out of question, unless your data is extremely important and worth a few thousand USD.
As an alternative you could write the hard-disk contents to an image file with dd, then browse that (large) file with a hex-editor or rather a custom program to search for a few specified file-types that you need. Some files like JPEG, TIFF, etc have easy to trace file headers and predictable file-endings, so these can be relatively easily extracted from that image file. You need to know what you are looking for.
Either way - there is no easy solution. For fast results you should consider a professional service provider for data recovery, and prepare for relevant costs. On the long term regular backups are the cheapest solution.