Be aware that the Messages App can use different protocols as determined by the messaging type...
iMessage (blue bubbles) supports attachment of images/pictures.
Cellular SMS messaging (green bubbles) uses your iPhone's Cellular connection - your iPad accessing these services via your iPhone using Apple's Continuity services. SMS, as a service, fundamentally does not support attachment of images/pictures. By contrast, MMS (if supported by your Cellular plan) when enabled in iPhone settings does support picture messaging. Be aware that MMS messages are often chargeable per message - and can add significantly to your Cellular phone bill.
To enable MMS on your iPhone (if required):
Settings > Apps > Messages > [Text Messaging] MMS - set to ON
Your iPad/iPhone will continue to communicate with a recipient using the last successful message exchange. Following release of iOS/iPadOS 26.x, some users have noted that existing message conversations have inexplicably switched from using iMessage (blue bubbles) to SMS (green bubbles). In such circumstances, starting a new conversation with a recipient from the Contact App should resume the conversation using iMessage - and with this, restore the ability to attach images to a message.