Well, I think you can save a workbook with a Chart Sheet to HTML. Please see the following sample code in this regard:
Workbook workbook = new Workbook();
Cells cells = workbook.Worksheets[0].Cells;
//Put a string into a cell
cells["A1"].PutValue("Region");
cells["A2"].PutValue("France");
cells["A3"].PutValue("Germany");
cells["A4"].PutValue("England");
cells["B1"].PutValue("Marketing Costs");
cells["B2"].PutValue(7000000);
cells["B3"].PutValue(5500000);
cells["B4"].PutValue(3000000);
Worksheet sheet = workbook.Worksheets[workbook.Worksheets.Add()];
//Set the name of the worksheet
sheet.Name = "Clustered Column";
sheet.IsGridlinesVisible = false;
//Create chart
int chartIndex = sheet.Charts.Add(ChartType.Column, 5, 1, 29, 10);
Chart chart = sheet.Charts[chartIndex];
//Add the nseries collection to a chart
chart.NSeries.Add("Sheet1!B2:B4", true);
//Get or set the range of category axis values
chart.NSeries.CategoryData = "Sheet1!A2:A4";
chart.NSeries.IsColorVaried = true;
//Set the legend position type
chart.Legend.Position = LegendPositionType.Top;
//Set properties of chart title
chart.Title.Text = "Marketing Costs by Region";
chart.Title.TextFont.IsBold = true;
chart.Title.TextFont.Color = Color.Black;
chart.Title.TextFont.Size = 12;
workbook.Worksheets.ActiveSheetIndex = 1;
workbook.Worksheets[0].IsVisible = false;
workbook.Save("C:\\Chart.html",FileFormatType.Html);
Also, for chart2Image feature, you may check the following user documentation link with details and sample code: