You could try FormatFactory, which I have been using for several years. That will convert just about any type of file to any other type and that applies to images too. It also has a 'file joiner' facility, which will convert and join in one operation but I find it quicker to let it convert the files first and then join the converted files as a separate operation. When you install it, the usual warnings apply, ensure nothing else is installed with it (set off tick boxes) but otherwise it is free of adware/malware (or else I would not use it!).
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