Welcome Jan! There are some very helpful people on these boards. As to your formation, the -ειδῶς ending is adverbial (the -ως is the clue): for the adjective you need -ειδής (which declines, of course). For the first part σαλπιγγο- would retain the omicron of the genitive, so we'd end up with *σαλπιγγοειδής "trumpet-like", "salpinx-shaped. This would be a perfectly normal kind of formation; there are probably hundreds of -ειδής compounds. The accent of the adverb follows that of the adjective (not predictable until you've mastered some quite complex rules, and even then ... -- best just learn the accents as you encounter them, after learning the basic rules). The corresponding noun is not actually εἴδωλον (an image), which does not lend itself to compound formation (as least not as the final element), but εἶδος, a shape or form.
Hope this helps.