feat: add image handling to image request for form-data

relay/helper/valid_request.go

 package helper
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"math"
@@ -150,6 +151,9 @@ func GetAndValidOpenAIImageRequest(c *gin.Context, relayMode int) (*dto.ImageReq
 			imageRequest.N = uint(common.String2Int(formData.Get("n")))
 			imageRequest.Quality = formData.Get("quality")
 			imageRequest.Size = formData.Get("size")
+			if imageValue := formData.Get("image"); imageValue != "" {
+				imageRequest.Image, _ = json.Marshal(imageValue)
+			}
 
 			if imageRequest.Model == "gpt-image-1" {
 				if imageRequest.Quality == "" {