I sympathize with your unease, but unfortunately we simply must translate it this way because of the limits of the English language. This has to do with the fact that English simply does not have the grammatical mood used in the Biblical Hebrew.
Languages signal the modality of a given verb based on the mood it is placed in (for instance, the indicative, subjunctive, conditional, etc.). In Biblical Hebrew, there is a mood not found in English, the cohortative. This mood is to be contrasted with the imperative. Generally speaking, the cohortative mood expresses the speaker's will, desires, or intentions, whereas the imperative mood is used to expresses the speaker's commands to another. In Gen 1:26, we have:
וַיֹּאמֶר אֱלֹהִים, נַעֲשֶׂה אָדָם בְּצַלְמֵנוּ
In this case, we have the Divine speaking in the 1st person common plural cohortative for the verb 'עשׂה'. This reading is reinforced when we consider the pronominal suffix 'נו-' ("our") appended to the word 'צלם' ("image"), once again indicating a plural subject (i.e. "we desire to make humanity in our image").
Therefore, since the verb in this verse is conjugated in the 1st person plural, the most faithful renderings into English will invariably indicate this by using "us" at this point.
As should be clear, the English rendering is simply a faithful translation of the Hebrew, and not at all contingent on doctrinal suppositions concerning a Trinity.