Make reference images useful in a technical inquiry
An image attached without a sentence is a question mark. Three labels turn it into evidence.
Images in a technical enquiry are usually attached and left to speak for themselves. They rarely do. The recipient does not know where the picture came from, which part of it you mean, or what you want said about it.
Three labels fix that, and they go in the message body, not in the image.
Three labels
Where it came from - Your own photograph, a supplier catalogue, a drawing, a website. This decides how much weight it carries. A picture from a catalogue shows what a product is supposed to look like; your photograph shows what arrived.
Which part of it you mean - "The thread at the top right", "the marking below the handwheel". If a picture has several features, say which one the question is about.
What you want established - "Is this the outlet for XF-25?" or "Is this marking the model designation?" An image without a question is decoration.
Four things not to do
A few practical points. Do not annotate over the only copy - send the clean image and describe the location in words, or send both. An arrow drawn across a marking can hide the thing being asked about.
Do not crop so tightly that the context disappears. A close-up of a thread with no surrounding body is hard to place, and the surrounding body is often what makes the identification possible.
And do not resize down to save space. A marking that is legible at full resolution may not be after compression, and that is usually the whole point of the picture.
Say when it is not the actual item
Finally, be explicit when an image is illustrative rather than of the actual item. "This is a similar valve from another supplier, to show the arrangement I mean" is a completely different statement from "this is the item we received", and a supplier will investigate them differently.
Where it came from, which part you mean, and what you want established.