I use OneUI in several applications, and have been sweating over drop-down buttons with images a couple of days.

The little documents that come up in searches indicates that this is straight-forward.

Anyway I found that it is straight-forward for any of the bootstrap themes, while any of the OneUI themes will not display an image next to the menu-item.

Server and designer updated to 9.0.1FP8, extlib 9.0.1.17.
Source:

                <xe:this.treeNodes>

                    <xe:basicContainerNode
                        image="/1_Day_selected_24.gif" label="testlabel 2">
                    <xe:this.children>
                        <xe:basicContainerNode
                            image="/1_Week_deselected_24.gif" label="1 week">
                            <xe:this.children>
                                <xe:basicLeafNode
                                    image="/1_Week_selected_24.gif"
                                    label="oneweek selected basic leaf" imageAlt="alt">
                                </xe:basicLeafNode>
                                <xe:pageTreeNode
                                    image="/1_Work_Week_deselected_24.gif" label="pagenode">
                                </xe:pageTreeNode>
                            </xe:this.children>
                        </xe:basicContainerNode>
                    </xe:this.children></xe:basicContainerNode>
                </xe:this.treeNodes>
            </xe:dropDownButton>